FIG. 1 shows a schematic configuration of a conventional mobile communication system. As shown in FIG. 1, the conventional mobile communication system includes a radio communication terminal MS, radio base stations BS, radio network controllers RNC and a core network CN. The radio communication terminal MS executes cell search to search for a connectable cell (radio base station) among multiple cells at the time of network entry or handover.
FIG. 2 shows an example of a process procedure of the cell search executed by the radio communication terminal MS in the mobile communication system shown in FIG. 1. In Step S11 of FIG. 2, the radio communication terminal MS detects reception power of a cell reference signal (CPICH (Common Pilot CHannel) of a BCH (Broadcast Control Channel in a UMTS (Universal Mobile Telecommunications System)) transmitted by a radio base station DS included in neighbor cell information (a neighbor list) Notified by radio base stations BS.
Thereafter, in step S12, the radio communication terminal MS determines whether or not a connection destination candidate cell (radio base station BS) is found, based on a result of comparison between a preset threshold and the reception power of the cell reference signal detected in Step S11. When the connection destination candidate cell is not found, the cell search is executed again in Step S1. On the other hand, when the connection destination candidate cell is found, the process moves to Step S13. Note that, when there is no connection destination candidate cell, the radio communication terminal MS is subjected to a process under out-of-service conditions.
In Step S13, the radio communication terminal MS attempts to access to the connection destination candidate cells in descending order of the reception powers of the cell reference signals. When the connection destination candidate cell does not have a sufficient channel capacity or when the connection destination candidate cell cannot provide the radio communication terminal MS with predetermined communication quality, the access is denied by the connection destination candidate cell.
Specifically, for accommodating the radio communication terminal MS, a cell is required to reserve a sufficient unused channel capacity and a required reception power (a reception power obtained by averaging an instantaneous fading variation and obtained in consideration of path loss caused by long-term so variation and shadowing variation).
When the access is denied by the connection destination candidate cell, the process moves to step S14. On the other hand, when the access is permitted by the connection destination candidate cell, the process moves to Step S15.
In Step S14, the radio communication terminal MS selects a cell (radio base station Bs) having the second largest reception power of the cell reference signal and attempts to access to the cell in Step S13. Meanwhile, in Step S15, the radio communication terminal MS executes access to the radio base station BS to which the access is permitted.
Meanwhile, as a mobile communication system using multiple frequency bands, a multiband mobile communication system has boon known. In the multiband mobile communication system, as shown in FIG. 3, frequency hands are provided while being spaced apart from each other to such an extent that propagation and diffraction characteristics vary among them. Moreover, transmission systems (for example, CDMA, TDMA and OFDMA) may be different among the frequency bands, and each radio base station may use multiple frequency bands.
FIG. 4 shows a network configuration example of the multiband mobile communication system. In FIG. 4, each of the radio network controller RNC, the radio base stations BS and the radio communication terminal MS can use three frequency bands f1, f2 and f3. Each of the radio base stations BS and the radio communication terminal MS includes three transceivers so corresponding to the three frequency bands f1, f2 and f3, and includes a frequency management function.
FIG. 5 shows a schematic configuration example of the radio communication terminal MS in the multiband mobile communication a system. FIG. 5 (a) shows a functional block configuration of the radio base station BS compatible with two frequency bands of an UHF band and a S band (hereinafter referred to as a “dual band”), and the radio communication terminal MS compatible with the dual band, in downlink. FIG. 5 (b) shows a functional block configuration of the radio base station BS compatible with only the UHF band, the radio base station BS compatible with only the S band, and the radio communication terminal MS compatible with the dual band, in the downlink.
Next, with reference to FIGS. 6 and 7, description will be given of an access control method conventionally proposed for the multiband mobile communication system. As shown in FIG. 6, the radio base station BS allocates the high frequency bands (f2 and f3) to the radio communication terminal MS having small path loss within a line-of-sight range or the like. Meanwhile, the radio base station BS allocates the low frequency band (f1) to the radio communication terminal MS in a poor propagation environment beyond the line-of-sight range or the like.
FIG. 7 is a flowchart showing the access control method conventionally proposed for the multiband mobile communication system. In Step S21k of FIG. 7, the radio communication terminal MS executes cell search for the radio base station BS using the highest frequency band among the radio base stations BS included in the neighbor list, and attempts to access to the found radio base station BS.
When the access is permitted, the ratio communication terminal MS is connected to the radio base station BS using the highest frequency band in Step S22k. Similarly, in Steps S21k-1 to S211, the cell search is executed for the radio base stations BS in descending order of the used frequency hands.
Thereafter, the radio communication terminal MS selects a cell which has provided the largest reception power among the cells (radio base stations BS) capable of accommodating the radio communication terminal MS, and accesses the selected cell (Steps S22k to S221).
As described above, in the conventional access control method in the multiband mobile communication system, the radio communication terminal MS accesses the radio base station BS using the high frequency band as long as the terminal MS can be connected thereto (Non-patent Documents 1 and 2, for example).
In other words, the low frequency band is reserved for a user in a very poor propagation environment. The low frequency band is better in propagation characteristics than the high frequency band. For example, the low frequency band produces smaller loss in diffraction or reflection, distance attenuation and instantaneous value variation of a signal received by the radio communication terminal than the high frequency band. Thus, it is desirable to reserve the low frequency band as precious resources.
If the low frequency band can be efficiently utilized, more radio communication terminals in a very poor propagation environment such as in a room, under the ground, behind a building and cell edge (cell boundary) can be served. This contributes not only to improvement in communication services and in profits of a carrier, but also consequently to improvement in utilization efficiency of frequency resources.    Non-patent Document 1: Hara, “A Multiband Mobile Communication System”, Shingakugihou, RCS2004-399, March 2005    Non-patent Document 2; Hara, Taira, “System Configuration of Multiband MC-CDM System”, Shingakugihou, RCS2004-400, March 2005